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Despite a fourth straight day of temperatures over 40 Celsius - a run not seen since 1908 - Serena Williams was untroubled in her straightforward 6-3, 6-3 win.

Shielded from the sun by a pink cap yesterday, the 32-year-old world number one wielded a killer serve as she overpowered Slovakia's 31st seed Daniela Hantuchova in 80 minutes, setting up a meeting with Serbia's Ana Ivanovic.

"It was hot but you have to play, you have to be ready and prepare yourself mentally. I'm excited to get through," Williams said.

It was Williams' 61st victory at the tournament, breaking the record set by 11-time champion Margaret Court up to 1975.

Williams' first Australian Open match win was on her debut aged 16 in 1998.

Asked which had been her favourite victories, she replied: "All the finals I was able to win."

The paucity of Australian success at Melbourne Park over the past four decades has not stopped organisers from pushing local battlers onto centre court, invariably for a thumping loss.

However, 120th-ranked wildcard Casey Dellacqua lived up to her Rod Laver Arena billing by reaching the fourth round, sending packed terraces of fans home with a warm, fuzzy and somewhat rare feeling.

The 28-year-old Dellacqua turned back the clock to upset a heat-affected Chinese Zheng Jie 6-2, 6-4 in the afternoon.

Zheng, who needed medical treatment for heat stress in worrying scenes, described how she felt sick and had palpitations.

The 2010 semi-finalist, playing during one of Melbourne's most severe heatwaves, said she couldn't focus and was hitting the ball randomly during her loss.

"I felt so hot, my mind wasn't working. I was looking at the ball, but I couldn't focus on it. Then I couldn't concentrate. This weather is very difficult for me," she said.

Zheng, 30, lay down and was rubbed with ice and had her pulse checked before continuing against the local favourite and eventually losing. Other players have fainted and vomited and a ball boy collapsed.

The Age newspaper said 1,000 spectators were treated for heat exhaustion in one day.

Three hours into the evening session, Dellacqua became the only Australian left in either of the men's and women's draws after 17th seed Sam Stosur was overhauled by former world number one Ivanovic in three sets 6-7 (8-10) 6-4, 6-2.

Like the higher-profile Stosur, Dellacqua has struggled with huge expectations in the country that produced Rod Laver, Ken Rosewall and Lleyton Hewitt, since making the fourth round of the 2008 Australian Open.

That breakthrough was followed by serious shoulder injuries, surgeries and further setbacks over the next few years and the Perth-born doubles specialist has had to graft in the minor tours to earn ranking points for another shot at the big-time.